ABSTRACT
The epizonal Mount Aetna caldera in central Colorado consists of the Mount Aetna pluton and the associated large-volume Badger Creek Tuff. New CA-TIMS U/Pb zircon ages are consistent with field relations that demonstrate that the Badger Creek Tuff was erupted during the incremental assembly of the ~34.9-34.3 Ma Mount Aetna pluton. The youngest dated portion of the pluton intrudes the ~34.5 Ma Badger Creek Tuff and yields a weighted mean 206Pb/238U age of 34.28 ± 0.06 Ma. This sample also contains 34.9 Ma antecrysts that date the onset of pluton emplacement, but none that are similar in age to the ignimbrite. Similarly, the Badger
Creek Tuff has no antecrysts that date the emplacement of the pluton. New major and trace element data are consistent with older studies, and suggest that the Badger Creek Tuff is not the fractionated equivalent of the Mount Aetna pluton. The disparity in age between these units also precludes this possibility. We interpret the eruption of the Badger Creek Tuff to be the result of a sudden and temporary spike in magma flux against the background low-flux assembly of the
Mount Aetna pluton.
INTRODUCTION
Two endmember hypotheses have been proposed to explain the spatial and temporal connections between intrusive rocks and associated large-volume (>400 km3) ignimbrites. The first suggests that large ignimbrites are the erupted fraction of even larger magma chambers that underwent differentiation and fractional crystallization in the upper crust (Bowen, 1928;
Bachmann and Bergantz, 2004; Hildreth, 2004), and that the unerupted portion of the chamber is preserved as plutonic rocks. The second hypothesizes that most plutons are incrementally assembled during periods of low magma flux, and that large volume ignimbrites
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(“supereruptions”) are erupted during periods of high magma flux, perhaps leaving very little in the plutonic rock record (Glazner et al., 2004; Tappa et al., 2011; Zimmerer and McIntosh, 2012;
Mills and Coleman, 2013).
Understanding the connections between intrusive rocks and large-volume ignimbrites is directly tied to deciphering the mechanisms that govern crustal growth and the processes responsible for supereruptions. Eruptions as small as 100-200 km3 have been tied to temporary changes in climate that directly affect the ability of humans to produce crops (Stothers, 2004), and the 2000 km3 75 ka eruption of Toba (Matthews et al., 2012) is thought to have reduced the human population to a few thousand (Ellis and Mark, 2013). Understanding the processes and rates at which large eruptible bodies are produced is important for anticipating and mitigating hazards associated with large eruptions.
To better understand the relationships between plutonic rocks and broadly contemporaneous volcanic rocks, it is important to recognize a general intrusive and eruptive sequence. Field relations can be verified with geochronology, which can be used to derive detailed intrusive and eruptive histories for magmatic centers. Because of its unambiguous field relations, the Mount Aetna caldera complex, which exposes the Mount Aetna pluton and the associated Badger Creek Tuff, is an ideal location to test the connections between plutonic and large-volume volcanic magmatism. Geochronologic data (Mills and Coleman, 2013) suggest that the pluton (34.95 ± 0.04 Ma) is somewhat older than the tuff (34.47 ± 0.05 Ma); however, the existing map for the caldera and local field relations indicate that the Mount Aetna pluton is resurgent into the tuff (Shannon, 1988; Mills and Coleman, 2013). These apparently contradictory observations might suggest that the Mount Aetna pluton was incrementally assembled across the period of time marked by eruption of the Badger Creek Tuff. Alternatively,
Hallman 2 because the field relations unambiguously demonstrate that at least part of the pluton is younger than the tuff, perhaps the U-Pb zircon data do not accurately date intrusion of the Mount Aetna pluton. Evaluating this possibility is critical for the interpretation of zircon ages from any pluton.
The traditional caldera model (Bowen, 1928; Smith and Bailey, 1968; Bachmann and
Bergantz, 2004; Hildreth, 2004) predicts essentially synchronous ages between the Badger Creek
Tuff and the Mount Aetna pluton, which would have been emplaced over a short period largely preceding eruption of the tuff. Alternatively, the flux-dependent hypothesis (Glazner et al., 2004;
Tappa et al., 2011; Zimmerer and McIntosh, 2012; Mills and Coleman, 2013) predicts that the
Mount Aetna pluton should have been incrementally assembled over perhaps several hundred thousand years that span a period of ignimbrite quiescence. In order to evaluate these two endmember hypotheses, we provide new high-precision CA-TIMS U-Pb zircon geochronology for the porphyritic Mount Aetna Quartz Monzonite where it intrudes the Badger Creek Tuff
(MPRM-DSC), a fine-grained dike that cuts the porphyritic Mount Aetna pluton (MA13-01), and new fractions of the fine-grained facies of the Mount Aetna pluton (MPRM-20) previously dated by Mills and Coleman (2013).
GEOLOGIC SETTING
The Mount Aetna caldera complex (Figure 1) is exposed in the southern portion of the late Eocene Mount Princeton batholith in the central Colorado volcanic field. The Mount Aetna caldera is structurally within the Sawatch Range block, which is adjacent to the Arkansas River valley along the main fault of the Rio Grande rift system (Mills and Coleman, 2013).
Magmatism at the Mount Aetna caldera began following Laramide shortening and prior to the onset of Rio Grande extension, which helped to expose the system through Neogene faulting and
Hallman 3 erosion (Epis and Chapin, 1975; Shannon, 1988). The calc-alkaline chemistry and spatially linear distribution of magmatism within the central Colorado volcanic field led most authors (Coney and Reynolds, 1977; Lawton and McMillan, 1999; Zimmerer and McIntosh, 2012) to attribute volcanism to the decoupling of the Farallon plate from the North American plate following flat- slab subduction, which brought volcanism ~1200 km inland from the trench; however, others
(Mutschler et al., 1987) dispute the subduction hypothesis and suggest that the magmatism reflects the early onset of Rio Grande rift tectonics.
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Figure 1. Simplified geologic map of the Mount Aetna caldera including sample locations
(modified from Shannon, 1988; Mills and Coleman, 2013).
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The Mount Aetna pluton is a quartz monzonite that intrudes the ~36.0-35.3 Ma Mount
Princeton batholith (Mills and Coleman, 2013). An earlier study (Mills and Coleman, 2013) reports a weighted mean CA-TIMS 206Pb/238U zircon age of 34.95 ± 0.04 Ma for the porphyritic
Mount Aetna Quartz Monzonite and 34.47 ± 0.05 Ma for the Badger Creek Tuff. A ring dike, a tuff dike, and a fine-grained phase of the Mount Aetna pluton (MPRM-20) were found to be the same age as the Badger Creek Tuff within uncertainty (Mills and Coleman, 2013). A portion of the porphyritic Mount Aetna pluton (MPRM-DSC) intrudes the Badger Creek Tuff, which is inconsistent with published geochronology (Mills and Coleman, 2013), motivating this study.
METHODS
U-Pb Geochronology
Samples were prepared for zircon separation with a jaw crusher and split into fractions for geochronology and geochemical analysis. The fraction for geochronology was further processed with a disc mill, and mineral separations were carried out using a water table, heavy liquids, and a magnetic separator. Zircon fractions were hand-picked using a binocular microscope. Select grains were thermally annealed for 48 hours at 850°C and chemically abraded for 8-16 hours in a pressure-dissolution vessel containing 6 M HCl at 180°C in order to remove damaged zones that were open to Pb diffusion (Mattinson, 2005). Fractions were picked and spiked with a 205Pb-233U-236U tracer and then dissolved in 29 M HF in a pressure solution vessel for approximately 90 hours at 220°C before conversion to a chloride salt. The separation of U and Pb was accomplished with HCl anion exchange column chromatography.
Isotopic ratios (Table 1) were measured with a thermal ionization VG Sector 54 mass spectrometer employing a Daly detector. Silica gel was used to load U and Pb onto single Re
Hallman 6 filaments, and U was run as an oxide. Corrections for initial Th/U disequilibrium were made using the method of Mattinson (1973), and whole rock trace element data from Mills and
Coleman (2013).
Elemental chemistry
Samples were prepared with a steel jaw crusher and manually split into representative aliquots for elemental chemistry. The aliquot for chemistry was powdered with a ceramic shatterbox and ignited at 950°C to expel volatiles (Lechler and Desilets, 1987). Major element analysis was performed on lithium metaborate/tetraborate flux discs. Trace element analysis was performed on pressed powder discs that were prepared using paraffin and a 25 ton press.
Elemental analysis was conducted by XRF in the Department of Geological Sciences (Tables 2 and 3).
RESULTS
U/Pb geochronology
New fractions of a fine-grained intrusion (MPRM-20) previously dated by Mills and
Coleman (2013) are generally consistent with the older data, but also include new, younger fractions ranging in age from ~34.7 to 34.3 Ma that yield a weighted mean 206Pb/238U age of
34.32 ± 0.03 Ma. A newly identified fine-grained dike that cuts the Mount Aetna Quartz
Monzonite (MA13-01) could be traced for approximately 200 m on a NNE-SSW bearing. It yields an age of 34.37 ± 0.04 Ma. Fractions are consistent in age except for an ~35.6 Ma antecryst that is similar in age to the Mount Princeton batholith (Mills and Coleman, 2013). It includes no fractions that are similar in age to the early period of emplacement of the porphyritic
Mount Aetna pluton at ~34.95 Ma (Mills and Coleman, 2013). A sample of the Mount Aetna
Hallman 7 pluton where it intrudes the Badger Creek Tuff (MPRM-DSC) yields a weighted mean 206Pb/238U age of 34.28 ± 0.06 Ma. It also includes fractions that are similar in age to the older phase of the
Mount Aetna pluton (MPRM-21) analyzed by Mills and Coleman (2013). We found no fractions with ages of ~34.6 to 34.4 Ma.
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Major and trace element geochemistry
The chemistry presented in this study is in agreement with earlier studies on the Mount
Aetna Quartz Monzonite and Badger Creek Tuff (Mills and Coleman, 2013; Shannon, 1988;
Toulmin and Hammarstrom, 1990; Campbell, 1994). All plutonic and volcanic samples contain approximately 64 to 66 wt% SiO2 (Table 2). Major element concentrations are similar for all analyzed units. Samples of the porphyritic (MA13-02, MPRM-21) and fine-grained (MPRM-20)
Mount Aetna Quartz Monzonite tend to have Zr/Hf ratios similar to the chondritic value of 36.3
(Sun and McDonough, 1989), as well as higher MgO, CaO and TiO2 (Mills and Coleman, 2013; this study). The Badger Creek Tuff (BCT13-02, MPRM-30) and a fine-grained dike (MA13-01) have relatively high Ce concentrations of ~130 ppm (Table 3) and elevated Zr/Hf ratios of approximately 42 (Mills and Coleman, 2013; this study).
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DISCUSSION
Field Relations
The porphyritic Mount Aetna Quartz Monzonite is unambiguously intruded by a fine grained dike (MA13-01) that we interpret as a feeder dike to the Badger Creek Tuff based on its size, age, and trace element chemistry. The Mount Aetna Quartz Monzonite from which MPRM-
DSC was sampled unambiguously intrudes the Badger Creek Tuff. The observed field relations are reconcilable if either zircon geochronology does not accurately date the age of emplacement, or if the Mount Aetna pluton was incrementally assembled over a period that spanned the eruption of the Badger Creek Tuff.
New U/Pb zircon ages
We assume that Pb loss has been mitigated by chemical abrasion (Mattinson, 2005) and interpret the emplacement age of intrusive units as the age of the youngest cluster of fractions.
We also assume that antecrysts represent assimilated or remobilized grains that yield insight into the extent of interaction between the sample unit and other magmas and wall rock. All presented dates are weighted mean 206Pb/238U ages.
New fractions of a fine grained unit of the Mount Aetna Quartz Monzonite (MPRM-20) are presented here. They are generally consistent with data from Mills and Coleman (2013), but also include new, younger fractions in addition to a tight cluster at 34.65 Ma. Our preferred emplacement age is 34.32 ± 0.03 Ma based on two young concordant fractions at that age; however, it is possible that these fractions represent grains that have experienced Pb loss, and that the older cluster is a more accurate emplacement age. One discordant fraction was discarded, and most young grains have low radiogenic Pb concentrations. We classify this unit as a member of the Mount Aetna pluton based on its low Zr/Hf ratio and its prolonged emplacement history. It
Hallman 12 is contemporaneous with the Badger Creek Tuff eruption, but its age distribution is consistent with the expected spread of an incrementally emplaced intrusive unit. The fact that it includes no antecrysts from the earliest recognized stage of pluton emplacement starting at ~34.9 Ma is surprising, since that suggests that it experienced limited interaction with the porphyritic phase of the Mount Aetna Quartz Monzonite system. Alternatively, this unit might be a member of the
Badger Creek Tuff system that remobilized zircon from the Mount Aetna pluton. In that case, the age distribution could represent mixing of rims and antecrystic cores in single fractions.
A new fine-grained sample (MA13-01) that intrudes the Mount Aetna Quartz Monzonite yields an emplacement age of 34.37 ± 0.04 Ma based on 5 concordant fractions that cluster at that age. A non-concordant fraction (F-3), an antecryst (F-7), and a fraction with a low radiogenic to common lead ratio (F-4) were omitted from the age reduction. This sample is approximately the same age as the Badger Creek Tuff within uncertainty. The fact that it includes no antecrysts from the pre-ignimbrite phase of pluton emplacement starting at ~34.9 Ma suggests that it was not comagmatic with the Mount Aetna Quartz Monzonite system and did not significantly interact with that unit.
The youngest analyzed increment of the Mount Aetna pluton is a sample (MPRM-DSC) that intruded the Badger Creek Tuff at 34.28 ± 0.06 Ma based on three concordant fractions that cluster at that age. Older (~34.9 Ma) fractions are interpreted as antecrysts from the pre- ignimbrite pluton. A discordant 34.13 Ma fraction was not included in the calculation of the emplacement age. The fact that this sample contains no fractions that are similar in age to the
Badger Creek Tuff suggests that it was not comagmatic with the Badger Creek Tuff system.
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Emplacement of the Mount Aetna Quartz Monzonite
Emplacement of the Mount Aetna pluton began with a porphyritic facies (MPRM-21) by
34.95 Ma (Mills and Coleman, 2013) and continued until the initiation of emplacement of the fine-grained facies of the pluton (MPRM-20) at ~34.7 Ma. The fine-grained facies is contemporaneous with the Badger Creek Tuff, and was emplaced from ~34.7 to 34.3 Ma.
Emplacement of the porphyritic Mount Aetna pluton appears to have resumed at ~34.3 Ma with the emplacement of the unit sampled for MPRM-DSC. The age range recorded by fractions of
MPRM-DSC (~700 ka) can be considered a minimum interval over which the Mount Aetna pluton was assembled.
The absence of any fractions from the youngest observed portion of the Mount Aetna pluton (MPRM-DSC) with ages comparable to the Badger Creek Tuff suggests that the Mount
Aetna Quartz Monzonite did not appreciably interact with the spatially-associated Badger Creek
Tuff magma.
Eruption of the Badger Creek Tuff and associated intrusions
The Badger Creek Tuff, a tuff dike, a ring dike, and two fine-grained dikes that cut the porphyritic Mount Aetna pluton are approximately the same age within uncertainty (Mills and
Coleman, 2013; this study). No fractions from these samples have been found that date the pre- ignimbrite intrusion of the Mount Aetna pluton. The absence of any antecrysts in the Badger
Creek Tuff system that record the emplacement of the Mount Aetna pluton beginning at about
34.9 Ma suggests that the Badger Creek Tuff did not appreciably interact with the Mount Aetna
Quartz Monzonite.
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Magmatic Evolution of the Mount Aetna caldera
Previous workers (Toulmin and Hammarstrom, 1990; Campbell, 1994; Zimmerer and
McIntosh, 2012) proposed that the Badger Creek Tuff is the extrusive equivalent of the Mount
Aetna pluton, but the nearly identical chemistries of the Mount Aetna Quartz Monzonite and the
Badger Creek Tuff suggest that the ignimbrite is not the fractionated equivalent of the pluton
(Zimmerer and McIntosh, 2012). Alternatively, the observed field relations are consistent with incremental assembly of the Mount Aetna pluton over a period that includes the eruption of the
Badger Creek Tuff. High-precision geochronology indicates that the Mount Aetna pluton was assembled over at least 700 ka, while the geochronology of the Badger Creek Tuff eruption or eruptions occurred within a duration too short to be quantified with available analytical precision. The absence of pre-eruption zircon grains in units associated with the Badger Creek
Tuff and the absence of syn-eruption zircon grains in the Mount Aetna pluton indicate that the pluton and the ignimbrite magmas did not significantly interact. A logical conclusion that follows from this observation is that the Mount Aetna pluton and Badger Creek Tuff belong to separate magmatic systems, and that the Badger Creek Tuff is not the fractionated equivalent of the Mount Aetna Quartz Monzonite. Instead, the Mount Aetna pluton was emplaced during a period of ignimbrite quiescence that was temporarily interrupted by the eruption of the Badger
Creek Tuff (Figure 2).
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Figure 2. Weighted mean 206Pb/238U ages with 2 uncertainties for units associated with the
Mount Aetna caldera. The bar encloses the weighted mean age of the Badger Creek Tuff based on 5 concordant fractions (Mills and Coleman, 2013).
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The significance of the fine-grained facies of the Mount Aetna pluton (MPRM-20) is more ambiguous. We interpret an emplacement age of 34.32 ± 0.03 Ma based on two young concordant fractions, and assume that older fractions are antecrysts that record the incremental emplacement of this unit. Its fine-grained texture is a notable departure from the porphyritic
Mount Aetna Quartz Monzonite, and indicates that emplacement conditions were different for this unit. We attribute this change to the development of Badger Creek Tuff magma system at about that time. This unit could represent the chilled margin from the waning stages of emplacement of the Mount Aetna Quartz Monzonite before the eruption of the Badger Creek
Tuff, since a decrease in the amplitude or frequency of temperature oscillation might have prevented the large rapakivi feldspars that characterize the Mount Aetna Quartz Monzonite from forming (Mills et al., 2011). This might yield insight into the different rates and periodicity of magma emplacement between plutons and ignimbrites. Alternatively, if generation of melt in the lower crust were diverted from assembly of the Mount Aetna pluton in the upper crust to formation of the Badger Creek Tuff magma, then flux to the Mount Aetna pluton would decrease, yielding smaller intrusive increments that could crystallize more quickly, resulting in a fine-grained equivalent.
Implications for crustal assembly
New geochronology reveals that the voluminous Badger Creek Tuff was erupted during a break in major pluton assembly. The temporal disconnect between the Mount Aetna pluton and the Badger Creek Tuff suggests that they belong to separate but related systems. Previous studies have found that the timing of pluton emplacement does not necessarily correspond to the timing of large-volume ignimbrite eruption (Glazner, 1991; Wilson and Charlier, 2009). Based on this observation, it has been hypothesized that plutons are assembled during periods of low magma
Hallman 17 flux and that ignimbrites are erupted during periods of high magma flux (Glazner et al., 2004;
Tappa et al., 2011; Zimmerer and McIntosh, 2012; Mills and Coleman, 2013). Based on the temporal disconnect and chemical similarity between the Mount Aetna Quartz Monzonite and the Badger Creek Tuff, we propose that a brief spike in magma flux against the background of low-flux pluton assembly led to the eruption of the Badger Creek Tuff. This implies that magma generation rates may rapidly change by orders of magnitude, which might yield insight into the processes by which melt is generated and conducted to the upper crust. An important implication of these conclusions is that under this model, a “supereruption” would not necessarily be preceded by the inflation of a large, detectable magma chamber.
CONCLUSIONS
The unambiguous crosscutting relations exposed at the Mount Aetna caldera coupled with high precision geochronology reveal a long-lived (~700 ka) emplacement history.
Agreement between field relations and geochronology reveals that there is no cause to doubt the accuracy of U-Pb zircon dating. New geochronology indicates that the Badger Creek Tuff was erupted during a break in the emplacement of the Mount Aetna pluton, which we attribute to a significant but temporary increase in magma flux. This temporal disconnect indicates that large ignimbrites are not the fractionated equivalent of associated plutonic rocks, and that traditional models of caldera formation may need to be reevaluated in favor of one in which magmatic style is controlled by magma flux.
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ACKNOWLEDGEMENTS
This project was supported by the Charles and Elaine Mims Fellowship, administered by the Department of Geological Sciences. Allen Glazner provided laboratory equipment and sup- plies. Lab training was provided by Ryan Frazer, Katie Wooton, Sean Gaynor, Tom Chapman,
Courtney Beck, and Scott Brinkley. Field work was completed with the assistance of Roger
Putnam, Sean Gaynor, and Josh Rosera. Discussions with Marcelaine Tanner, John Barefoot, and
Scott Brinkley improved this project. Ryan Mills shared data, advice, and his years of experience with Aetna. Drew Coleman provided all of the above, as well as invaluable guidance and tireless patience.
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